Method of transmitting synchronized channel in radio transmitter

ABSTRACT

The invention relates to a method of transmitting a synchronized channel in a radio transmitter and to a radio transmitter. The method comprises transmitting normal radio bursts on a normal channel asynchronously. In the invention, synchronized timing is obtained, synchronized radio bursts (SB) are formed, and a synchronized radio burst is transmitted in the place of a normal radio burst (NB). The length of the synchronized radio burst (SB) is at most half of the length of the normal radio burst (NB). The transmission of the synchronized radio burst (SB) is in synchronization with the obtained synchronized timing.

This application is the national phase of international applicationPCT/F199/00247 filed Mar. 25, 1999 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to a method of transmitting a synchronized channelin a radio transmitter, where normal radio bursts are transmitted on anormal channel asynchronously.

BACKGROUND OF THE INVENTION

Cellular radio networks comprise applications which require that asubscriber terminal or some other corresponding radio receiver receivessynchronized radio signals from various base stations. Such applicationsinclude different methods of locating subscriber terminals. An exampleof such locating methods is an OTD (Observed Time Difference) methodbased on time differences detected in the reception of signals. In thismethod a terminal equipment measures differences in times of arrivals ofsignals transmitted by base stations. The method requires that the basestations transmit signals at the same moment, in other wordssynchronously, or otherwise data is required on the differences insynchronization (Real Time Difference, RTD) between the base stations ifthe base stations are not synchronized. The location is carried outbased on this data. This method is described in greater detail inFinnish Patent Application 954,705.

Several systems, such as the GSM system, are not synchronized or theyare not synchronized sufficiently accurately so that the signals couldbe used in the location according to the OTD method. In the GSM system,normal channels are divided both on a time division (TDMA, time divisionmultiple access) and frequency division (FDMA, frequency divisionmultiple access) basis. A radio transmitter thus uses a specific timeslot on a predetermined frequency for transmitting a normal physicalchannel. In the GSM system, the base stations transmit radio bursts of anormal channel asynchronously, which means that the transmissionsbetween the base stations are not coordinated such that each basestation would transmit a radio burst simultaneously. Further, theaforementioned synchronization differences between the base stationschange over time. Therefore the OTD method cannot be used for locationwithout continuous measurement of the synchronization differences.Measurement of the synchronization differences produces more signallingand causes additional error in the accuracy of the location.

One suggested solution is to synchronize all the radio transmitters witheach other by means of a satellite-based locating system (globalpositioning system, GPS), in which case a GPS receiver would beinstalled at each base station. This arrangement may cause problems inthe GSM system since the system utilizes hierarchical clocks. This meansthat a base station controller guiding a base station obtains timingfrom higher network elements and delivers it to the base stations. If aGPS receiver were used for the timing of the base station transmission,the entire timing of the GSM system would be confused.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to develop a method and an apparatusimplementing the method which solve the aforementioned problems. This isachieved with a method of the type described in the introduction, whichis characterized by obtaining synchronized timing; forming synchronizedradio bursts, the length of which is at most half of the length of anormal radio burst; transmitting a synchronized radio burst in the placeof a normal radio burst such that the transmission of the synchronizedradio burst is synchronized with the obtained synchronized timing.

The invention also relates to a radio transmitter comprising a channelcodec for forming a normal channel; a burst former for forming normalradio bursts; a multiplexer for assigning to each burst the moment forits transmission.

The radio transmitter according to the invention is characterized inthat it also comprises a clock for obtaining synchronized timing; thechannel codec is arranged to form a synchronized channel; the burstformer is arranged to form synchronized radio bursts, the length ofwhich is at most half of the length of a normal radio burst; themultiplexer is arranged to insert a synchronized radio burst in theplace of a normal radio burst such that the transmission of thesynchronized radio burst is synchronized with the obtained synchronizedtiming.

The preferred embodiments of the invention are disclosed in thedependent claims.

A basic idea of the invention is that a radio burst normally used by aradio transmitter is at least halved so that the obtained synchronizedradio burst can always be inserted flexibly in the place of the normalradio burst. The expression in the place of means that the normal radioburst is replaced in principle, i.e. the burst that is to be actuallytransmitted is not necessarily replaced but the synchronized burst istransmitted during the time slot in which it would be possible inprinciple to transmit the normal radio burst.

The method and the radio transmitter according to the invention provideseveral advantages. Synchronized signals can be transmitted to areceiver without a need to make any changes in the general timingstructure. For example the GSM system does not require changes in theTDMA frame structure. The structure of the synchronized signals can beoptimized according to the needs of the intended use, such as a locatingmethod.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be described in greater detail inconnection with preferred embodiments, with reference to theaccompanying drawings, in which

FIG. 1 shows an example of the structure of a cellular radio networkemploying the invention,

FIG. 2 shows the structure of a transceiver,

FIG. 3 shows synchronized radio bursts according to the invention andthe moments when they are transmitted at four different base stations,

FIG. 4 shows two different alternatives of transmitting a synchronizedradio burst in the place of a normal radio burst,

FIG. 5 shows the structure of a synchronized radio burst,

FIGS. 6 and 7 are flowcharts illustrating the implementation of themethod according to the invention,

FIG. 8 shows the positioning of a synchronized radio burst with paddingbits in the place of a normal radio burst.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be used in different radio transmitters. The examplesdescribe the use of the invention in a cellular radio network. Withreference to FIG. 1, the structure of a typical cellular radio networkwill be described. FIG. 1 only contains the blocks that are essentialfor explaining the invention, but it is clear for those skilled in theart that a conventional cellular radio network also comprises otherfunctions and structures, which do not have to be described in greaterdetail herein. The examples describe a cellular radio network employingtime division multiple access (TDMA) without restricting the inventionthereto, however.

A cellular radio network typically comprises a fixed networkinfrastructure, i.e. a network part 128, and subscriber terminals 150,which may be fixed, located in a vehicle or portable hand-held terminalequipments. The network part 128 comprises base stations 100. Severalbase stations 100 are controlled in a centralized manner by a basestation controller 102 communicating with them. A base station 100comprises transceivers 114. A base station 100 typically comprises 1 to16 transceivers 114. For example in the TDMA radio system, onetransceiver 114 typically provides radio capacity for one TDMA frame,i.e. eight time slots.

The base station 100 comprises a control unit 118, which controls theoperation of the transceivers 114 and a multiplexer 116. The multiplexer116 places the traffic and control channels used by several transceivers114 onto a single transmission link 160.

The transceivers 114 of the base station 100 have a connection to anantenna unit 112, which realizes a bidirectional radio link 170 to asubscriber terminal 150. The structure of frames to be transmitted onthe bidirectional radio link 170 is accurately determined and it isreferred to as an air interface.

FIG. 2 shows in greater detail the structure of a transceiver 114. Thefunctions at the reception will be described first. A receiver 200comprises a filter blocking frequencies outside a desired frequencyband. A signal is thereafter converted onto an intermediate frequency ordirectly to baseband, and the signal in this form is sampled andquantized in an A/D converter 202.

An equalizer 204 compensates for interference caused by multipathpropagation, for example. A demodulator 206 extracts from the equalizedsignal a bit stream, which is transferred to a demultiplexer 208. Thedemultiplexer 208 separates the desired part from the received bitstream into logical channels. This function is based on the structure ofthe received bit stream, which consists of radio bursts placed in timeslots, forming a physical channel.

A channel codec 216 decodes bit streams of different logical channels,i.e. it decides whether a bit stream consists of signalling data, whichis transmitted to a control unit 214, or speech, which is transmitted240 to a speech codec 122 in the base station controller 102. Thechannel codec 216 decodes possible channel coding, such as block codingand convolutional coding, deinterleaves possible interleaving, anddecrypts the encryption used over the radio path.

The control unit 214 carries out internal control tasks by controllingdifferent units mainly on the basis of control received from the basestation controller 102.

The functions at the transmission will be described next. The data to betransmitted is channel-coded, interleaved and encrypted in the channelcodec 216. A burst former 228 adds a training sequence and a tail to thedata arriving from the channel codec 216. A multiplexer 226 assigns toeach burst its physical channel. A modulator 224 modulates digitalsignals onto a radio frequency carrier. This function is analogous,wherefore it requires a D/A converter 222.

A transmitter 220 comprises a filter restricting the bandwidth. Thetransmitter 220 also controls the output power of the transmission. Asynthesizer 212 provides different units with required frequencies. Thesynthesizer 212 comprises a clock, which may be locally controlled orcontrolled in a centralized manner from some other place, for examplethe base station controller 102. The synthesizer 212 creates thenecessary frequencies by means of a voltage-controlled oscillator, forexample.

As shown in FIG. 2, the structure of the transceiver can further bedivided into radio-frequency parts 230 and a digital signal processorwith its software 232. The radio-frequency parts 230 comprise thereceiver 200, the transmitter 220 and the synthesizer 212. The digitalsignal processor with its software 232 comprises the equalizer 204, thedemodulator 206, the demultiplexer 208, the channel codec 216, thecontrol unit 214, the burst former 228, the multiplexer 226 and themodulator 224. Conversion of an analogue radio signal into a digitalsignal requires an A/D converter 202 and, correspondingly, theconversion of a digital signal into an analogue signal requires a D/Aconverter 222.

The base station controller 102 comprises a group switching field 120and a control unit 124. The group switching field 120 is used forswitching speech and data and for connecting signalling circuits. Thebase stations 100 and the base station controller 102 form a basestation system 126, which also comprises a transcoder 122. Thedistribution of functions between the base station controller 102 andthe base station stations 100 as well as their physical structure mayvary in different implementations. The base stations 100 typicallymanage the implementation of the radio path as described above. The basestation controller 102 typically manages the following things:configuration of traffic channels, frequency hopping control, paging ofsubscriber terminals, power control, quality control of active channels,and handover control.

The transcoder 122 is usually located as close to a mobile servicesswitching centre 132 as possible, because this allows speech to betransmitted between the transcoder 122 and the base station controller102 in a cellular radio network form, which saves transmission capacity.The transcoder 122 converts different digital speech coding modes usedbetween a public switched telephone network and a radio phone network tomake them compatible, for example from a 64 kbit/s fixed network form toanother form (such as 13 kbit/s) of the cellular radio network, and viceversa. The control unit 124 performs call control, mobility management,gathering of statistical data, and signalling.

As shown in FIG. 1, a circuit-switched connection can be set up from thesubscriber terminal 150 to a telephone 136 connected to the publicswitched telephone network (PSTN) 134 via the mobile services switchingcentre 132. The cellular radio network may also employ a packet-switchedconnection, for example 2+phase packet transmission, i.e. GPRS (GeneralPacket Radio Service), of the GSM system.

The structure of the subscriber terminal 150 can be described by meansof the representation of the structure of the transceiver 114 shown inFIG. 2. The structural elements of the subscriber terminal 150 arefunctionally identical to those of the transceiver 114. The subscriberterminal 150 also comprises a duplex filter between the antenna 112 andthe receiver 200 and the antenna and the transmitter 220, user interfaceparts and a speech codec. The speech codec is connected to the channelcodec 216 via a bus 240.

FIG. 3 shows how transmissions of four different base stations BTS 1,BTS 2, BTS 3, BTS 4 are not synchronized with each other. Each basestation transmits its normal bursts NB at instants that differ randomlyfrom one another. According to the invention, each base station receivestiming, which is described in FIG. 3 by successive bursts SYNCHRONIZEDBURSTS. Timing is received from a clock, which is for example a GPSreceiver 180 connected to the control unit 118 of the base station 100as shown in FIG. 1. The control unit 118 forwards the received timing tothe transceivers 114.

In the invention, a special synchronous channel is formed in the channelcodec 216. In principle the synchronous channel is placed on a normalphysical channel. The number of physical channels available is acompromise. For example in the OTD locating method, the more frequentlysynchronous signals are transmitted the more often the subscriberterminal 150 is able to receive them and to carry out more measurements,which improves the accuracy of the location. On the other hand, thisconsumes more traffic capacity of the system. The example shown in FIG.3 utilizes one frequency, i.e. all the eight time slots of one TDMAframe, i.e. eight physical traffic channels. If the traffic capacity isto be consumed as little as possible, only one time slot can be used totransmit synchronized bursts, for example time slot ‘one’ of a broadcastcontrol channel (BCCH), in which case the subscriber terminal 150 alwaysknows the location of the synchronized bursts after it has received onenormal synchronization channel burst (SCH). In order that the capacityof an uplink physical channel corresponding to a downlink synchronizedchannel would not be wasted, the capacity can be used to forwardsignalling data, such as measurement results of the subscriber terminal150, to the base station 100.

A preferred embodiment utilizes the normally unused capacity fortransmission of synchronized radio bursts. For example when a radiotransmitter is in a mode of discontinuous transmission (DTX) and nonormal radio bursts are being transmitted, it is possible to transmitinstead synchronized radio bursts, on the basis of which the subsicberterminal 150 is able to determine its location, for example.

Another method of making the operation more effective is to transmitsynchronized radio bursts by means of only a part of the capacity of aphysical channel. In such a situation the synchronous bursts arerepeated according to a predetermined sequence, for example in everythird time slot of the physical channel.

The physical channel to be used for transmitting the synchronizedchannel can be indicated to the subscriber terminal 150 on a controlchannel, such as the broadcast control channel (BCCH).

The burst former 228 is arranged to form synchronized radio bursts SB.The length of a synchronized radio burst SB is at most half of thelength of a normal burst NB in order that the synchronized burst SB canalways be inserted in the place of the normal burst NB. The multiplexer226 is arranged to insert the synchronized radio burst SB in the placeof the normal radio burst NB in such a way that the transmission of thesynchronized burst SB is synchronized with the timing obtained from theclock 180.

FIG. 3 shows timing in the form of possible synchronized burstsSYNCHRONIZED BURSTS. A vertical line has been drawn from the start andend of each such burst to describe the instant a synchronous burst SBcan be transmitted at each base station BTS 1 TIMING, BTS 2 TIMING, BTS3 TIMING, BTS 4 TIMING. The synchronized bursts SB transmitted by eachbase station start and end at exactly the same instant.

It can be seen from FIG. 3 that in a preferred embodiment the timingshappen to match at base station BTS 1, whereupon two synchronized burstsSB can be transmitted in the place of a normal burst NB. The burstformer 228 is arranged to form successive synchronized bursts SB, whichthe multiplexer 226 inserts in the place of the normal burst NB sincethey fit there. On the other hand, this embodiment can also be avoidedif receiving two synchronized bursts during one time slot causesproblems in the subscriber terminal 150, in which case only one of thesynchronized bursts is transmitted.

At base station BTS 2, the timings differ from one another exactly halfa time slot, and therefore it is possible to transmit two synchronizedbursts SB in the place of the normal burst NB.

However, in the most common situation the timing obtained by the basestation 100 from the network and the timing obtained from the clock 180do not match. In such a case it is possible to transmit only onesynchronized burst SB in the place of the normal burst NB as shown inFIG. 3 with base stations BTS 3 and BTS 4. As the figure shows, everyother synchronous burst SB would extend to two normal bursts NB, whichis not desirable.

FIG. 5 illustrates the structure of a synchronized burst SB. In the sameway as a normal burst a synchronized burst must also comprise tail bitsTB both at the beginning and end of the burst. These bits are usedduring a guard period when the transmitter increases the power to therequired transmit power and thereafter lowers it to the idle state. Thetail bits are usually set to zero.

As shown in FIG. 4, a synchronized burst SB can be inserted in the placeof a normal burst NB in two different manners. The first manner is shownin the middle in the figure. The synchronized burst SB shown therein isa special burst of FIG. 5, the length of which is at most half of thenormal burst NB. Nothing else is transmitted in this time slot besidesthe synchronized burst SB.

The second manner is illustrated in FIG. 4 at the bottom. The burstformer 228 is arranged to form a burst that is equal in length to anormal radio burst NB, and a synchronized burst SB is inserted therein.The part of the formed burst that does not belong to the synchronizedburst SB is filled with predetermined padding bits PAD. This embodimentprovides an advantage that the transmission time of the burst does nothave to be changed, but only the content thereof is altered.

As shown in FIG. 5, the synchronized burst SB comprises at least apredetermined bit pattern TS. Usually this bit pattern is a trainingsequence which is also known to the receiver and which can be searchedin the equalizer 204. By comparing this known training sequence to thesignal that is actually received it is possible to estimate what kind ofdistortions have accumulated in the signal over the radio path. When thereceiver receives the synchronized burst SB it also obtains accuratetiming, since the transmission moment of the burst is determined to bethe same at different base stations, unlike in the case of normal burstsNB. For the purpose of locating methods the structure of a known bitpattern can be optimized suitably.

In a preferred embodiment a synchronized burst also comprises otherinformation INFO as shown in FIG. 5. The information may contain thelocation coordinates COORD of the base station 100. Timing offset OFFSETcan also be transmitted in the information field INFO. In this case theoffset refers to the time difference between the transmission moments ofthe ideal synchronized radio burst and the actual synchronized radioburst. In reality, the transmission moment of the synchronized burst SBcan be adjusted with the accuracy of maybe only one bit or one fourth ofa bit, in which case the offset indicates the difference from the exactcorrect transmission moment. The information may further include otherinformation OTHER INFO, and the information can also be combinedCOORD+OFFSET in a desired manner.

To obtain the most accurate possible timing the training sequence TSshould be as long as possible. Therefore some or even all of theinformation INFO can be transferred to padding bits PAD, so that thetraining sequence TS can be continued to the place of the informationINFO. Since the position of the synchronized burst SB varies, sometimesthe information INFO would be placed before and sometimes after thesynchronized burst SB. In such a case the subscriber terminal 150 mustbe able to select the correct place from which the information INFO isdecoded.

FIG. 8 shows how a synchronized radio burst SB is inserted with paddingbits PAD in the place of a normal radio burst NB. This figureillustrates the implementation of the alternative shown lowermost inFIG. 4. The tail bits TB are naturally situated at the beginning and endof the burst. They are followed by padding bits PAD, which surround thetraining sequence TS and the information INFO.

The invention is preferably implemented by means of software and itrequires changes in an accurately restricted area of the software of thedigital signal processor 232 in the transceiver 114 of the base station100. The invention further requires that a radio transmitter obtainssynchronized timing for example from the clock 180.

The implementation of the method according to the invention in a radiotransmitter is further illustrated with reference to the flowcharts ofFIGS. 6 and 7. The method starts in block 600. In block 602 the methodproceeds to the next time slot. In block 604 it is checked whether thelogical channel to be transmitted in the time slot is normal orsynchronized. In block 606 normal radio bursts are transmittedasynchronously on a normal channel. In block 608, a synchronized burstformed according to the invention is transmitted. In block 610 it ischecked whether the method is to be continued. If not, the execution ofthe method is terminated in block 612. If it is continued, the processproceeds to block 602, where the processing of the next time slot isstarted.

Block 608 is described in greater detail in FIG. 7. The implementationbegins in block 700. Synchronized timing is obtained in block 702. Next,it is checked in block 704 whether it is time to transmit a synchronizedburst. If not, the process moves back to block 702 where the clock ischecked. This is repeated until it is time to transmit the synchronizedburst. When it is detected after the checking carried out in block 704that it is time to transmit a synchronized burst, the method proceeds toblock 706. In block 706 it is checked whether a sufficient part of thetime slot is left for the transmission of the synchronized burst. Ifnot, the method proceeds to block 712. If a sufficient part of the timeslot is left, the process moves to block 708 where synchronized radiobursts SB are formed, the bursts having a length of at most half of thelength of a normal radio burst. Next, in block 710 the synchronizedradio burst is transmitted in the place of a normal radio burst suchthat the transmission of the synchronized burst is synchronized with theobtained synchronized timing. The last step is block 712 where theexecution of block 608 is terminated.

Even though the invention is described above with reference to theexample according to the accompanying drawings, it is clear that theinvention is not restricted thereto but it can be modified in severalways within the scope of the inventive idea disclosed in the appendedclaims.

1. A radio transmitter comprising: a channel codec configured to form anormal channel; a burst former configured to form normal radio bursts; amultiplexer configured to assign to each radio burst a moment for itstransmission; and a clock configured to obtain synchronized timing,which synchronized timing defines the coordination between thetransmission of radio bursts from at least two different base stationseach including at least one radio transmitter; wherein the burst formeris arranged to form synchronized radio bursts, a length of each of thesynchronized radio bursts is at most half of a length of a normal radioburst, and the multiplexer is arranged to insert a synchronized radioburst in place of the normal radio burst such that transmission of thesynchronized radio burst is synchronized with the obtained synchronizedtiming.
 2. The radio transmitter according to claim 1, wherein the burstformer is arranged to form at least two successive synchronous radiobursts and the multiplexer is arranged to insert at least one of the atleast two successive synchronous radio bursts in place of the normalradio burst.
 3. The radio transmitter according to claim 1, wherein theburst former is arranged to form a burst having a length equal to alength of the normal radio burst, said burst comprising at least onesynchronized radio burst.
 4. The radio transmitter according to claim 3,wherein the burst former is configured to place predetermined paddingbits in a part of the burst that does not belong to the synchronizedradio burst.
 5. The radio transmitter according to claim 1, wherein theburst former is configured to place a predetermined bit pattern in thesynchronized radio burst.
 6. The radio transmitter according to claim 5,wherein the predetermined bit pattern is a training sequence.
 7. Theradio transmitter according to claim 1, wherein the channel codec isarranged to place in the synchronized radio burst information includingat least one of the location coordinates of the radio transmitter and anoffset.
 8. The radio transmitter according to claim 7, wherein theoffset is the time difference between transmission moments of an idealsynchronized radio burst and an actual synchronous radio burst.
 9. Theradio transmitter according to claim 1, wherein the multiplexer isarranged to place the synchronized radio burst in a time slot.
 10. Theradio transmitter according to claim 1, wherein the channel codec isconfigured to use at least one normal physical channel for thesynchronized channel.
 11. The radio transmitter according to claim 10,wherein the radio transmitter is configured to indicate on a controlchannel physical channels to be used for the transmission of thesynchronized channel.
 12. A The radio transmitter according to claim 1,wherein the radio transmitter is arranged to receive signaling data fromchannels in a direction of reception corresponding to synchronizedchannels in a direction of transmission.
 13. The radio transmitteraccording to claim 1, wherein the clock is a GPS receiver.
 14. The radiotransmitter according to claim 1, wherein the radio transmitter isarranged to transmit a synchronized radio burst when the transmitter isin discontinuous transmission.
 15. The radio transmitter according toclaim 1, wherein the radio transmitter is arranged to use only a part ofa capacity of a normal channel for transmission of synchronized radiobursts.
 16. The radio transmitter according to claim 1, wherein thechannel codec is arranged to further form a synchronized channel.